The present invention relates to a fuel reforming apparatus which generates mixed vapor by heating raw fuel such as methanol or etc. and reforms the mixed vapor to desired fuel such as a gas containing rich hydrogen by supplying the mixed vapor to a reformer in the fuel reforming apparatus. Especially, this relates a control device for the fuel reforming apparatus.
As one example of the aforementioned fuel reforming apparatus, a fuel reforming apparatus, which generates reformed gas which mainly contains hydrogen gas reformed from methyl alcohol (or methanol) and water, is known. This fuel reforming apparatus has a copper alloy or so as a catalyst. If an activating temperature of the catalyst is, for example, approximately 280xc2x0 C. (degree Celsius), when the temperature of the catalyst is below the above-mentioned value, the methanol is not sufficiently reformed and rather much methanol remains in the reformed fuel gas. Moreover, since a reforming reaction of methanol is an endothermic reaction, heat is supplied from an outer part of the catalyst in order to maintain the catalyst at the same temperature and to promote the fuel reforming reaction.
As a method of heating the catalyst, not only a heating by a burner is known, but a method of generating heat by oxidation and transferring the heat to the reformer is also known. The latter method can be called in another way a method by a partial oxidizing reaction. For example, methanol vapor and air are mixed, and hydrogen is generated by oxidizing this mixed gas under a catalyst. The heat generated in this oxidation process is used to promote the above-mentioned fuel reforming reaction. Accordingly, the heat absorbed by the endothermic reaction can be compensated by using this partial oxidizing reaction. It is not necessary to give heat from the outside, because a heating value and a heat absorbing value can be balanced. That is, it is only possible that the temperature of the fuel reforming apparatus can be maintained constant by balancing the heat values with the reforming and oxidizing reactions. It is, however, not possible that the temperature of the fuel reforming apparatus is set to a target temperature.
This means that it is necessary to heat from the outside to set the appropriate temperature of the reformer for the fuel reforming reaction or the activation of the catalyst. In order to achieve the above-mentioned object, the raw fuel mixed by methanol and water becomes the mixed vapor having a predetermined temperature by burning heat of a burning device, and the mixed vapor is supplied to the reformer.
If the above-mentioned fuel reforming apparatus is used as means for supplying fuel gas for a fuel cell, it is necessary to control the reaction in the fuel reforming apparatus on the basis of a fluctuation of a load of the fuel cell. That is, it is necessary to increase a quantity of the reformed raw fuel gas when the load of the fuel cell increases, or it is necessary to decrease a quantity of the reformed raw fuel gas when the load of the fuel cell decreases. In order to increase or decrease a quantity of the reformed raw fuel gas, it is necessary to increase or decrease a quantity of the raw fuel gas mixed by the methanol and water which is supplied to the reformer. Consequently, it is necessary to increase or decrease a heat value which is consumed in order to generate the mixed vapor of the methanol and the water having the predetermined temperature.
One example of control devices controlling such as the above-mentioned way is disclosed in Japanese Laid-Open Patent Application No. 7-296834. The control device is for a plant in which a fuel reforming apparatus is heated by a burner. By detecting an inlet and an outlet temperatures of the fuel reforming apparatus and an inlet and an outlet temperatures of a burner in the fuel reforming apparatus, a quantity of a fuel gas flow and an oxidizing agent is controlled on the basis of these temperatures. The stable burning of the burner in the fuel reforming apparatus is then maintained.
As proposed in the aforementioned Japanese Laid-Open Patent Application, if an inlet and an outlet temperatures of the fuel reforming apparatus and an inlet and an outlet temperatures of the burner in the fuel reforming apparatus is detected, not only a condition of a reforming reaction and a temperature of the reforming reaction can be known, but a stable condition of the burning can also be known in the case that unused raw fuel gas is burned by the burner. Accordingly, the stable burning of the burner can be maintained by controlling on the basis of the detected values. The control is, however, insufficient when the quantity of raw fuel supplied to the fuel reforming apparatus fluctuates.
These fuel reforming apparatuses are usually used as a fuel source for a fuel cell or etc., as written in the above-mentioned Patent Application, and a quantity of raw fuel supplied to the fuel reforming apparatus changes according to a required quantity of raw fuel. Since the raw fuel is supplied at the lower temperature than the activating temperature of a catalyst for a reforming reaction of the raw fuel, it is necessary to heat the raw fuel and make it vapor. That is, it is necessary to increase or decrease a quantity of fuel for burning on the basis of the fluctuation of the raw fuel. There are, however, several obstructing factors as mentioned below. One is a time delay from the indication time to change a quantity of raw fuel until the real time to supply the raw fuel to a vapor device in the fuel reforming apparatus. Another is a time delay from the indication time to change a quantity of fuel for burning until the real time to supply the fuel for burning to the burning device. Furthermore, the other is a time delay until attaining to the heat value corresponding to the quantity of the fuel for burning, and the other factor is a relative discrepancy between the real condition and surroundings according to the fluctuation of the quantity of raw fuel or fuel for burning. Therefore, when the heat of raw fuel is controlled on the basis of the fluctuation of temperatures caused by the fuel reforming reaction or the burning of the fuel, it is not possible to obtain the quantity of raw fuel having the target temperature, and it may occur that the burning device melts in some parts and suffers damage.
It is thus one object of the present invention to solve the aforementioned problems. Another object of the invention is to provide a control device for a fuel reforming apparatus improving transient responsibility of a control for heating raw fuel.
A fuel reforming apparatus comprises a heating device, and the heating device heats raw fuel to a predetermined target temperature. A control device for the fuel reforming apparatus comprises a raw fuel quantity determinator for determining a quantity of raw fuel supplied to the heating device and a target temperature setting means for setting the target temperature of the raw fuel on the basis of the quantity of raw fuel determined by the raw fuel quantity determinator.
If a quantity of raw fuel supplied to the heating device increases, the latent heat of the raw fuel increases when the raw fuel vapors. Since the target temperature of the raw fuel is set high, the temperature of the heated raw fuel is prevented from lowering even if a heat value taken as vaporizing latent heat increases. Accordingly, the target temperature of the raw fuel can be achieved.
The heating device comprises a burning device and a vaporizing device. The burning device oxidizes fuel for burning with air, and the vaporizing device vaporizes the raw fuel. The control device further comprises an air quantity controller for controlling a quantity of air supplied to the heating device based on the quantity of raw fuel determined by the raw fuel quantity determinator.
When a quantity of raw fuel is less, a quantity of air supplied to the heating device is less. On the contrary, when a quantity of raw fuel is more, a quantity of air is more. If a target temperature of raw fuel is set low because the quantity of raw fuel is less, then, a heat value for heating the air and increasing the temperature of the air or a heat value which the air takes out becomes less because the quantity of the air is less. On the contrary, if a target temperature of raw fuel is set high because the quantity of raw fuel is more, a heat value for heating the air and increasing the temperature of the air or a heat value which the air takes out becomes more because the quantity of raw fuel is more. Consequently, the raw fuel does not under-heat or over-heat.
It is also available that the control device further comprises a detector for detecting a physical value indicating the quantity of raw fuel heated by the heating device and a correcting means for correcting the target temperature on the basis of the physical value detected by the detector.
Since the control device has the above-mentioned detector and the correcting means, the quantity of air supplied to the heating device or the target temperature of raw fuel is corrected on the basis of the quantity of raw fuel flow or so. Accordingly, a deviation of heat value in the heating device, a deviation heat exchanging value, or a producing deviation of functional parts such as a valve included in the fuel reforming apparatus does not influence the temperature of the raw fuel.
If the control device further comprises a heat value correcting means for correcting a heat value of the heating device on the basis of the heat capacity of the heating device, the heat value of the heating device is controlled by considering the heat capacity of the heating device. Thus, the raw fuel is precisely heated and is not insufficiently heated or over heated.